The influence of water and electrolyte replacement therapy and treatment with antibiotics against acute (3 to 5 day) intestinal radiation deaths in rats exposed to an acute lethal dose of x radiation was studied. The administration of sterile 0.9% solution containing either NaCl or a mixture of NaCl, Na lactate, and KCl during the critical 2 to 5 day period after irradiation had little, if any, protective effect. However, treatment with antibiotics during the same period definitely increased the number of survivors. This antibiotic -effect was enhanced significantly when NaCl, but not when equiosmolar glucose, was a constituent of the infusion solution given concurrently with the drug. The results strongly suggest that microorganisms may play a more prominent role than dehydration in the genesis of acute intestinal radiation death in the rat, but that dehydration must also be taken into account in the treatment of acute intestinal radiation injury. A comparison between whole-body and abdomen-only irradiated animals given a dose of 910 r showed that the therapeutic measures used were about equally effective against acute intestinal radiation death in the former as in the latter cases. It was observed, however, that third-day deaths, which were so prominent in whole-body irradiatedmore » animals, were virtually absent in cephalic one-half-shielded, abdomen-irradiated animals. it was further observed that these therapeutic measures were ineffective against deaths occurring primarily as a consequence of radiation-induced injury to the hematopoietic system, as evidenced by the high incidence of deaths during the second postexposure weeks (the period usually associated with bone marrow radiation syndrome) in whole-body but not in partial-body irradiated animals. A large number of 5 day survivors, protected against acute intestinal radiation death by the combined saline and antibiotic therapy after 1000 r of x irradiation to the abdomen only, died during the subsequent week. These deaths, which occurred in bone marrowshielded animals, apparently resulted from radiation injury sustained by the small intestine and not by any other abdominal viscera in the radiation field. (auth)« less